If libupnp is installed in a non-standard location we must rely on the
include path provided by $(pkg-config --cflags libupnp). Relative to the
path given from that command no prefix must be used to find the respective
files.

libupnp 1.6.24 added a few badly designed macros which break the MPD
build:

 https://sourceforge.net/p/pupnp/code/ci/8177a4195a4d4a5d6cb095eaf4ca5b65ac446381/

To work around this, we disable our emulation functions (from
714011c8) on this libupnp version.

Closes #163

Closes #155

This eliminates some overhead, because the compiler doesn't need to
consider these functions throwing.

With "ok==false", newly created threads may quit instantly.

libupnp provides seconds, not whatever time unit is used by
std::chrono::steady_clock.

When uri_apply_base() was moved from db/upnp/Util.cpp to
util/UriUtil.cpp, the parameter order was changed, however without
swapping the parameters in the ContentDirectoryService constructor.

Necessary for "final" on gcc 4.6.

Eliminate class LibUPnP and move the code to ClientInit.cxx.  Its
initialization function can be called multiple times, but
UpnpRegisterClient() is called at most once.

Allow calling UpnpGlobalInit() multiple times.